This invention relates generally to measurement of optical fibers, and in particular to measurement of optical fiber propagation time measurement.
It is well known by those skilled in the art that the time required for light to propagate through the length of an optical fiber is dependent upon the refractive index of the fiber and the wavelength of the light being transmitted. For a commercial grade fused silica optical fiber found in a conventional fiberoptic network, the refractive index is roughly 1.49 and the propagation time is roughly 5.0 ns/m (nanoseconds per meter). There are many well known methods for determining the length of optical fibers and measuring the time of light propagation down a fiber. Instruments such as time-domain reflectometers, for example, are capable of measuring both length and propagation time and displaying a graph of the resultant measurement. Typically, conventional methods are expensive and rely on either high-speed counters or high-speed digital sampling to make the measurements, and therefore there is a limit to the measurement resolution, which in turn contributes to inaccuracies. For example, sampling with a 100-megahertz clock results in 10 nanoseconds between samples. If the propagation time is approximately five ns/m, the sampling uncertainty of ± one clock count means that accuracy of a length measurement is ± two meters. For measurement of long fibers, this may not be a problem. But for short fibers, an accuracy of ± two meters may be unacceptable.
It would be desirable to provide a measurement technique having high accuracy for short (<500 meters) optical fibers, and without the expense, complexity and power consumption of known methods.